aDepartment of Biological Sciences, University of Notre Dame, South Bend, IN 46556;bDepartment of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN 37996;cDepartment of Applied Ecology, North Carolina State University, Raleigh, NC 27695;

aDepartment of Biological Sciences, University of Notre Dame, South Bend, IN 46556;iInstitute on the Environment, University of Minnesota, St. Paul, MN 55108;jDepartment of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108

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Significance

The biological consequences of climate change are determined by the responses of individual species and interactions among species. Hybridization, or interbreeding between related species, is an interaction that affects how species evolve in response to environmental change. Here we provide evidence that climatic warming has caused a geographic shift of a butterfly hybrid zone and that strong selection and/or genetic incompatibilities maintain species boundaries during this movement. Through simulations, we show that as climate change progresses, the rate and geographic configuration of future hybrid zone movement will vary across space and time. This geographic variation in future hybrid zone movement may lead to divergent ecological and evolutionary outcomes, and thus has implications for local conservation and management.

Abstract

Climate-mediated changes in hybridization will dramatically alter the genetic diversity, adaptive capacity, and evolutionary trajectory of interbreeding species. Our ability to predict the consequences of such changes will be key to future conservation and management decisions. Here we tested through simulations how recent warming (over the course of a 32-y period) is affecting the geographic extent of a climate-mediated developmental threshold implicated in maintaining a butterfly hybrid zone (Papilio glaucus and Papilio canadensis; Lepidoptera: Papilionidae). These simulations predict a 68-km shift of this hybrid zone. To empirically test this prediction, we assessed genetic and phenotypic changes using contemporary and museum collections and document a 40-km northward shift of this hybrid zone. Interactions between the two species appear relatively unchanged during hybrid zone movement. We found no change in the frequency of hybridization, and regions of the genome that experience little to no introgression moved largely in concert with the shifting hybrid zone. Model predictions based on climate scenarios predict this hybrid zone will continue to move northward, but with substantial spatial heterogeneity in the velocity (55–144 km/1 °C), shape, and contiguity of movement. Our findings suggest that the presence of nonclimatic barriers (e.g., genetic incompatibilities) and/or nonlinear responses to climatic gradients may preserve species boundaries as the species shift. Further, we show that variation in the geography of hybrid zone movement could result in evolutionary responses that differ for geographically distinct populations spanning hybrid zones, and thus have implications for the conservation and management of genetic diversity.

Data deposition: Demultiplexed RADseq reads generated in this study are available through NCBI’s Sequence Read Archive associated with Bioproject (PRJNA431486, SRA: SRP131962). All metadata and scripts associated with analyses in this study have been deposited on DRYAD (https://doi.org/10.5061/dryad.5vn76).

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